Method for bonding solid propellants to rocket motor casing

ABSTRACT

The reliability of propellant-case bond adhesion is improved by chemically bonding the propellant binder to the insulator of the rocket motor casing during the propellant cure cycle. Bonding is effected by treating a surface of an uncured elastomeric insulator with a polyfunctional material such as unsaturated alcohols, acids, amines, or isocyanates, curing the insulator, placing uncured propellant against the treated surface, and then curing the propellant in place. Excellent adhesion of nitrocellulose-containing propellants to butadiene-styrene type insulators is demonstrated using undecylenic alcohol as the surface treating material.

United States Patent [191 Dehm [451 Feb. 13,1973

[54] METHOD FOR BONDING SOLID PROPELLANTS TO ROCKET MOTOR CASING [75]Inventor: Henry C. Dehm, Salt Lake City,

Utah

[73] Assignee: Hercules Incorporated, Wilmington,

Del.

[22] Filed: May 2, 1967 [21] Appl. No.: 637,037

[52] US. Cl ..264/3 R, 102/103, l49/94;95

[51] Int. Cl. ..C06b 21/02 [58] Field of Search ..149/109, 96-100,149/94, 95; 264/3; 102/103 [56] References Cited UNITED STATES PATENTS2,877,504 3/1959 Fox ..264/3 2,985,055 5/1961 McMichael... ..264/3 X3,021,748 2/1962 Miller ..264/3 X 3,213,173 10/1965 Cobb ..264/3 X3,215,028 11/1965 Pitchford et a1.

3,250,829 5/1966 Wall 3,381,614 5/1968 Ratz et al ..149/l09X PrimaryExaminer-Carl D. Quarforth Assistant Examiner-Stephen J. Lechert, Jr.Attorney-Michael B. Keehan [57] ABSTRACT 4 Claims, No Drawings METHODFOR BONDING SOLID PROPELLANTS TO ROCKET MOTOR CASING This inventionrelates to bonding solid propellants to the lining of rocket motor casesand, more particularly, to an improved method for chemically bondingsolid propellants to an elastomeric insulator material.

In the propellant field and, more particularly, in solid rocketry, it isoftentimes necessary to bond the propellant grain to an insulator, as,for example, to the elastomeric lining of a motor case. Unless thepropellant grain is securely held in the motor case throughout motorfiring, nozzle blockage, unpredictable variations in burning surface,overpressure, and even case rupture can occur. It is therefore necessaryto maintain between the propellant and the internal insulation a bondwhich has sufficient strength to prevent propellant separation duringhandling, storage, and firing of the motor.

The major problem in obtaining good adhesion between the propellant andthe insulator concerns the bonding of the dissimilar materials used inthe propellant grain and the insulator. Various materials and systemshave been used to achieve this bonding. For example, in one system thepropellant is held in place by nylon loops attached to the insulatorsurface, and in another system fingers of propellant are embedded in athick film of epoxy resin applied to the insulator surface. Althoughthese systems provide a reasonable degree of adhesion, the propellantand insulator bond is still susceptible to rupture due to its mechanicalnature and additionally, in some cases, due to softening of the epoxyresin by the propellant plasticizer. Moreover, because such systems areextremely complex, implementation is involved, time consuming, andexpensive. Therefore, there is a great need for a simple, inexpensive,and reliable method for bonding a solid propellant to an insulatorsurface.

Now, in accordance with the present invention, it has been found thatthe reliability of the bonding can be increased and hence thepossibility of motor failure due to propellant-case bond separationsignificantly reduced by chemically bonding the insulator surface to thepolymeric binder of the propellant during the normal cycle for curingthe propellant. More specifically, the present invention relates tochemically bonding a solid propellant to the elastomeric insulator of arocket motor casing which comprises (a) treating a surface of an uncuredelastomeric insulator with a polyfunctional material containing at leastone functional group chemically reactive with the elastomer during cureof the elastomer and at least one other functional group chemicallyreactive with the propellant binder during cure of the propellant; (b)curing the insulator; (c) placing uncured propellant against the treatedinsulator surface; and (d) curing the propellant in place.

Elastomeric materials which can be utilized as the rocket casinginsulator in accordance with this invention are any of the well-knownvulcanizable elastomers or rubbers. Particularly preferred are thestyrene--butadiene rubbers, polybutadiene, poly(2,3-dimethylbutadiene),natural rubber, and the like, which can, if desired, contain suchconventional filler materials as silica, carbon black, boric acid,asbestos, and the like.

Any propellant formulation which can be cured to give a solid rocketpropellant is suitable for use in the invention. Particularly preferredare the nitrocellulosecontaining propellants, as, for example, the olderand well-known double base type as well as the modern high energycomposite modified double base propellants that contain solid oxidizersand metal fuels. Nonnitrocellulose propellants employing such binders ascross-linked carboxy-terminated polybutadiene, polyisobutylene,butadiene-acrylic acid copolymer or butadiene-acrylic acid-acrylonitrileterpolymer, polyurethanes (isocyanate cross-linked long-chain diols),and the like can also be utilized in the process of the invention.

As stated above, a surface of the uncured insulator is treated with apolyfunctional material which contains groups which are l. chemicallyreactive with the elastomer during curing of the elastomer, and

2. chemically reactive with the propellant binder during curing of thepropellant.

Typical functional groups which chemically bond to elastomers duringcuring or vulcanization of the elastomer comprise allyl, hydroperoxy,mercapto, thiomercapto, thiuram, thioureido, thiazoyl, guanidino,mercaptothiazolyl, dithiocarbamyl, trithiocarbamyl, thiocarboxy,carbamyl, and like groups. Suitable functional groups which chemicallybond with the propellant binder during propellant cure will vary, ofcourse, depending upon the particular polymer employed as the propellantbinder. When nitrocellulose is the binder polymer, isocyanate groups areparticularly useful since they react directly with the hydroxyl groupsof the nitrocellulose chain to give urethane linkages. Other groups suchas hydroxyl, mercapto, primary or secondary amino and carboxyl groupswhich require the presence of cross-linking agents bearing isocyanate,epoxide, imine, and like functional groups during cure, are alsosuitable. Where the binder polymer is derived from a carboxy-terminatedprepolymer and a crosslinking agent, the functional group can be primaryor secondary amino, carboxyl, epoxy, or aziridino and is ideally primaryamino or carboxyl, and when the binder polymer is a polyurethane, thefunctional group can be hydroxyl, primary, or secondary amino,isocyanate, etc. Exemplary of polyfunctional materials which meet theabove requirements and are particularly useful in the process of theinvention include the allylically unsaturated alcohols, amines,carboxylic acids and isocyanates such as undecylenic alcohol,undecylenic amine, undecylenic acid, undecylenic isocyanate, crotonicalcohol, crotonic amine, crotonic acid, crotonic isocyanate, oleicalcohol, oleic amine, oleic acid, oleic isocyanate, linoleic alcohol,linoleic amine, linoleic acid, linoleic isocyanate, and the like.

In the method of the present invention, the surface of the uncuredinsulator is treated with the polyfunctional material as such or as asolution in a volatile solvent using any of the known techniques forapplying a coating of a solid or liquid to the surface of anothermaterial, as by brushing, spraying, and the like, and the treatedinsulator cured by vulcanizing, as, for example, by heating at 145 to325F. under pressures up to 45 p.s.i. for l to 2 hours. The propellantis then placed against the treated surface, as by casting if thepropellant is, for example, a slurry of nitrocellulose fuel and oxidizerin a plasticizer and the propellant cured, preferably at from to F. for5 to 12 days.

The following examples further illustrate, but are not means to limit,the invention. All parts and percentages therein are by weight unlessotherwise indicated. In these examples the insulator was 4.5 X 8 X 0.06inch sheets of uncured silica-filled styrene--butadiene rubber stock,and the propellant formulation was a homogeneous slurry containing 7.0percent nitrocellulose, 25.0 percent nitroglycerin, 18.0 percentaluminum powder, 45.0 percent cyclotetramethylenetetranitramine (180p.particle size), 4.0 percent polyethyleneglycol adipate-tolylenediisocyanate cross-linker, 1.0 percent 2-nitrodiphenylamine stabilizer,and 0.005 percent dibutyl tindiacetate catalyst.

EXAMPLE 1 T One surface of the insulator sheet stock was brushcoatedwith undecylenic alcohol to give a 1-2 mil coating and the coated stocksealed in an aluminum envelope and vulcanized immediately in a heatedplate press for 90 minutes at 325F. and 45 p.s.i. pressure. Aftervulcanization, the cured stock was removed from the envelope, washedwith methanol to remove any unreacted undecylenic alcohol, and thendried at 212F. for 1 hour. The propellant slurry was cast against thetreated surface of the cured stock in a peel test mold and cured inorder to evaluate the adhesion of the insulator surface to thepropellant using a 90 peel test. In this test the cured rubber wasplaced in a three-piece mold with the treated side up and the moldfilled to a depth of 0.5 inch with the propellant slurry. The mold wasthen sealed in a polyethylene bag and the bag containing the mold heatedin a 120F. oven for 12 days, after which time it was cooled to roomtemperature and disassembled. The propellant surface of the moldedinsulator-propellant slab was then bonded to a one-fourth inch plywoodsheet and the insulator surface bonded to fiberglass cloth with anepoxide adhesive, thus giving a plywood-propellant-insulator-glass clothsandwich. Inch-wide parallel cuts were then made lengthwise through thefiberglass-coated insulator to give 1 inch test strips which were peeledthrough a 90 angle at 75F. and a crosshead speed of 2 inches per minuteon an lnstron tensile tester. The peel test samples failed 100 percentin the propellant whereas test samples of a control, processed in thesame manner except that the uncured insulator stock was not coated withthe alcohol, failed 100 percent at the propellant-insulator interface.

EXAMPLE 2 The procedure of Example 1 was repeated except that in thisexample just before casting the propellant slurry against the treatedsurface of the cured insulator,

an extra 25 mg. of dibutyl tindiacetate was applied as an acetonesolution to each 200 sq. cm. of the treated insulator surface and theacetone permitted to evaporate at room temperature. The peel testindicated percent failure in the propellant.

EXAMPLE 3 The procedure of Example 1 was repeated except that in thisexample the cured insulator, after washing with methanol and drying, wascoated with a l-mil film of ol meth lene ol hen lisoc anate a lied bbrus hin g. Th e 90 pe tes indihated l08 percen l failure in thepropellant.

The above examples demonstrate that excellent adhesion between theinsulator and the propellant can be obtained using the method of thepresent invention. Although the invention has been exemplified using across-linkable propellant formulation, the invention is not so limitedand is applicable to either cross-linked or noncross-linked propellants.Likewise, the invention is not limited to propellants containingnitrocellulose as the binder polymer and is applicable to any propellantwhich cures via a chemical reaction, including those propellantscontaining polymeric binders derived from carboxylic acids such aspolybutadiene dicarboxylic acid, polyisobutylene dicarboxylic acid, andthe like, binders derived from long-chain diols, long-chain diamines,and the like, etc.

What I claim and desire to protect by Letters Patent l. A method forchemically bonding a solid propellant to an elastomeric insulator for arocket motor casing which comprises a. applying to a surface of anuncured elastomeric insulator a'coating of a polyfunctional materialcontaining at least one functional group chemically reactive with theelastomer during cure of the elastomer and at least one other functionalgroup chemically reactive with the propellant binder during cure of thepropellant,

b. curing the insulator,

c. placing an uncured propellant against the coated insulator surface,and

d. curing the propellant in place.

2. The method of claiml wherein the elastomeric insulator isbutadiene--styrene rubber and the propellant is anitrocellulose-containing propellant.

3. The method of claim 2 wherein the polyfunctional material isundecylenic alcohol.

4. The method of claim 3 wherein the nitrocellulose propellant is aslurry which is cast against the coated insulator surface and curing ofthe propellant is carried out in the presence of a polyisocyanate and acuring catalyst.

1. A method for chemically bonding a solid propellant to an elastomericinsulator for a rocket motor casing which comprises a. applying to asurface of an uncured elastomeric insulator a coating of apolyfunctional material containing at least one functional groupchemically reactive with the elastomer during cure of the elastomer andat least one other functional group chemically reactive with thepropellant binder during cure of the propellant, b. curing theinsulator, c. placing an uncured propellant against the coated insulatorsurface, and d. curing the propellant in place.
 2. The method of claim 1wherein the elastomeric insulator is butadiene--styrene rubber and thepropellant is a nitrocellulose-containing propellant.
 3. The method ofclaim 2 wherein the polyfunctional material is undecylenic alcohol.